The present invention relates generally to methods of designing molds. More specifically, the invention relates to methods of designing molds for orthopedic instruments and implants.
The traditional approaches to creating a shaped metal object include the lost wax and sand casting methods. Both casting methods rely on creating a mold of the object, either with a sand cavity or with a wax mold, then pouring in a liquefied metal. The metal fills the mold cavity and cools. Once the metal solidifies, the mold can be broken and the part removed. Alternatively, metal objects can be machined into shapes from a billet. For cast parts, secondary machining is usually necessary to meet tolerances and/or to achieve surface finishes and the like. However, machining operations require material removal and can be inefficient and costly.
Metal injection molding (MIM) allows net-shape or near net-shape metal components to be made in high volumes without expensive machining costs. Referring to FIG. 1, the MIM process starts in step 10 with mixing a metal powder and a polymeric binder to create a material that can be injected into a mold cavity. The binder provides the mixture with the appropriate rheological properties for handling, molding and later ease of removal from the die. The mixture is then heated in an injection mold barrel to an appropriate consistency and injected into a mold cavity to take the shape of the mold. The injection molding machines can be the same ones as used for plastic injection molded parts. Once the mold has been formed, the green part is ejected from the mold in step 12. The binder is then removed from the green part, typically by introducing the part to a solvent or by burning out the polymer. The next step 14 is sintering the part in an oven to partially melt the metal particles. Temperatures typically reach about 1000 degrees Celsius during sintering. During this stage, the part may shrink 15-20 percent as the metal particles consolidate. After sintering, the part has a density very close to that of a traditionally formed part. Finally, in step 16, the sintered part is machined to its final shape.
One embodiment of the final machining steps of the prior art is shown in FIGS. 2 and 3. FIG. 2 illustrates a schematic partial sectional view of a cutting block 304 with slitting saw blade 306 inserted to its maximum depth Slitting saw blades may enter the face of cutting block 304 to form a precisely sized slot for the surgical saw blade. However, as can be seen in FIG. 2, the curvature of the blades results in incomplete removal of the metal in the slot, illustrated at 307. In an attempt to clean up the slot, blade 306 may be inserted from the other side of cutting block 304. This second machining step is illustrated in FIG. 3. As can be seem, a “dragon's tooth” 405 is still present that must be removed by another operation. Other operations include using a wire EDM to finish the part, filing the part, milling, etc. These operations all add extra cost and time to the final product.
Among the advantages of metal injection molding are the greater design flexibility and lower machining costs. Whereas traditional machining methods rely on the removal of material from a part, MIM parts can be built to near net shape and later machining costs can be lowered or eliminated. All the benefits and design constraints of plastic metal injection molding essentially carry over to MIM.
Depending on the tolerances required for the final part, further machining may be necessary. In general, larger objects produced with MIM need much more subsequent machining than smaller parts, limiting MIM's applicability to smaller objects. For example, a typical tolerance for an unfinished MIM'ed part is ± 5/1000″ per inch, but a cutting slot tolerance for an orthopedic cutting block may be ± 1/1000″ per inch, so the finished MIM part must sometimes be machined to meet tolerances. Similar tolerances may be needed for implant components such as femoral stems, tibial baseplates, knee arthroplasty femoral components and other orthopedic implants.
Although MIM is becoming widely used, many products still need additional machining operations due to an inability to meet the fine tolerances needed for some applications. This is especially true for orthopedic cutting blocks and implants. Therefore, there is a continuing need to improve the design of molds used in a MIM or casting process.